This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7 119 from an application for ULTRASONIC DISPERSION DEVICE FOR SILICA GLASS SOL earlier filed in the Korean Industrial Property Office on the 31st of Aug. 1999 and there duly assigned Ser. No. 36610/1999.
1. Field of the Invention
The present invention relates to an apparatus for ultrasonically dispersing silica sol used in a process of manufacturing a silica glass by a sol-gel method.
2. Description of the Prior Art
For the manufacture of optical fiber, which is a high-speed and low-loss data transmission medium, both methods of directly drawing an optical fiber from a liquid material, for example, a double crucible process, and methods of drawing the optical fiber from a rod-shaped preform are known.
Among methods of drawing an optical fiber from a preform, various methods respectively using different preform formation processes are known, for instance, a method such as a modified chemical vapor-phase deposition (MCVD) process. In accordance with this method, a preform is formed by depositing a vapor-phase material on the inner or outer surface of a substrate tube. A method is also known in which a preform is formed by molding a core rod and an over-jacketing tube, and interconnecting the rod and tube to form the preform. Both the substrate tube of the modified chemical vapor-phase deposition process and the over-jacket tube, which are made of silica glass, are formed in the form of tubes, typically by using a sol-gel method.
In a general method of manufacturing a silica glass tube by the sol-gel method, a silica glass tube is manufactured using a mixing process, a dispersing process, a molding process, a de-molding process, a drying process, and a sintering process. An example of this method is disclosed in U.S. Pat. No. 5,240,488, to Chandross, entitled Manufacture Of Vitreous Silica Product Via A Sol-Gel Process Using A Polymer Additive.
In the mixing and dispersing procedures, a starting material is mixed with deionized water and an additive such as a dispersing agent so that the starting material is uniformly dispersed in the deionized water, thereby forming a uniform sol. The starting material may include silicon alkoxide or fumed silica. In particular, the dispersing procedure, which is adapted to uniformly disperse the start material in the deionized water, is a very important process in determining the uniformity of the final silica glass product.
In the molding procedure, the sol produced in the mixing and dispersing procedures is poured into a mold having a proper shape, for gelation. To the sol is added a binder and a gelling agent to aid in forming particle-to-particle bonds. The mold, which is used to mold a silica glass tube, for example, a substrate tube or an over-jacket tube, has a construction including a cylindrical portion and a central rod portion received in the cylindrical portion.
In the de-molding procedure, the gel molded to have a tube shape in the molding procedure is removed from the mold. This procedure may be carried out in a water tank to prevent the gel tube from being damaged.
In the drying procedure, the gel tube removed from the mold is dried using a drying means with a constant temperature and humidity chamber, and then subjected to a thermal treatment at a low temperature in order to remove or decompose residual molecular water and any organic materials present in the gel tube. And then, the gel tube is heated in an atmosphere of Cl gas to remove metallic impurities and hydrides.
In the sintering procedure, the formed body made via the drying procedure is sintered so that it is glassified. As a result, a desired silica glass product is obtained. This procedure is carried out by heating the dried and impurity-removed gel to a temperature of 1,350xc2x0 C. to 1,400xc2x0 C. in a sintering furnace in an atmosphere of He gas.
The dispersing procedure is carried out by a special ultrasonic dispersion device. Conventional ultrasonic dispersion devices may be classified as direct type or indirect type, the direct type class being divided into direct-static type and direct-dynamic type devices.
A conventional ultrasonic dispersion device of the above mentioned direct-static type includes a water bath with the top being opened, and a rod-shaped ultrasonic vibrator with a probe. In a dispersion process using this conventional ultrasonic dispersion device, the probe of the ultrasonic vibrator is in direct contact with a sol contained in the water bath. The ultrasonic vibrator is moved left and right or forward and rearward to disperse the sol in the water bath by ultrasonication.
A conventional ultrasonic dispersion device of the direct-dynamic type includes a dispersion chamber having a sol inlet and a sol outlet, and an ultrasonic vibrator with a probe positioned in the dispersion chamber. In a dispersion process using this ultrasonic dispersion device, a sol is supplied into the dispersion chamber through the sol inlet while the ultrasonic vibrator operates. The sol is dispersed by ultrasonic waves generated from the probe in the dispersion chamber. After the dispersion, the resultant sol is outwardly discharged from the dispersion chamber through the sol outlet.
With the constructions of the above ultrasonic dispersion devices, since the dispersion is carried out by a direct contact between the sol and the probe, the time needed for the dispersion is shortened, and the dispersion efficiency is increased. However, the direct contact between the sol and the probe results in an erosion of the probe, thereby producing impurities. Since the produced impurities are introduced into the sol, the purity of sol is lowered, thereby degrading the quality of the resultant silica glass.
A conventional ultrasonic dispersion device of the indirect type includes a water tank filled with water, an ultrasonic vibrator disposed on the bottom of the water tank, and a container floated in the water bath and filled with a sol. Ultrasonic waves generated from the ultrasonic vibrator are transmitted to the water in the water bath and the sol in the container, so that the sol is dispersed by the ultrasonic waves transmitted through the water.
With the construction of this conventional ultrasonic dispersion device of the indirect type, since the dispersion is indirectly carried out through water, no impurity is introduced into the sol. However, the time needed for the dispersion is extended. Furthermore, a variation in dispersion degree may occur depending on the positional level of the sol within the container.
Other examples of the conventional art involving ultrasonication are seen in the following U.S. Patents. U.S. Pat. No. 4,861,615, to Toki et al., entitled Silica Glass Formation Process, describes a process for forming silica glass articles using a sol gel method. The patent describes ultrasonication of a sol with simultaneous stirring.
U.S. Pat. No. 6,060,523, to Moffett et al., entitled Continuous Process For Preparing Microgels, describes a process in which a sol is turbulently mixed and aged as the sol passes through an elastically deformable vessel such as a pipe or tube. An ultrasonic vibrator is used to deform the walls of the vessel for the purpose of removing deposits.
U.S. Pat. No. 6,063,894, to Phipps et al., entitled Process For Purifying Polymers Using Ultrasonic Extraction, describes a process in which polymer pellets suspended in a cavitable liquid pass through a J-tube and are ultrasonicated by means of transducers externally mounted on the J-tube.
It is therefore an object of the present invention to provide an improved apparatus for ultrasonication of a silica sol.
A further object of the invention is to provide an ultrasonication apparatus which shortens the time needed for the dispersion procedure.
A yet further object is to prevent the introduction of impurities into the sol during ultrasonication.
Another object of the present invention to achieve a more uniform dispersion of silica sol.
In order to accomplish the above objects, the present invention provides an apparatus for dispersing a silica sol. The apparatus of the present invention includes a sol feeder filled with a sol; a sol container for receiving and containing a ultrasonically dispersed sol; a medium tank filled with a liquid-phase ultrasonic medium; an ultrasonic vibrator for generating ultrasonic waves within the tank; and a sol pipe for providing a sol feeding path connecting the sol feeder and the sol container to each other, the sol pipe having a portion submerged under the ultrasonic medium filled in the medium tank. A pump for pumping sol through the sol pipe may also be provided.